1. Field of the Invention
This invention is directed to both an additive process for manufacturing printed circuit boards and the printed circuit board produced thereby.
2. Description of the Prior Art
Over the past few decades, printed circuit boards have become the predominant, if not substantially exclusive, vehicle for mounting and interconnecting electronic components, e.g. resistors, capacitors, integrated circuits and the like, in order to manufacture any desired electronic circuit.
Basically, a printed circuit board consists of a sheet of rigid insulating substrate such as phenolic, glass impregnated epoxy or the like, having a pre-defined pattern of thin metallic--usually copper--foil cunductive paths (so-called "traces") appearing on one or both sides of the substrate. These traces collectively define all the electrical interconnections among all the components and are routed between appropriate locations on the board.
Generally, each component is electrically connected to its associated trace(s) through one or more corresponding mounting holes drilled completely through the substrate. Specifically, the leads of each component are first inserted into corresponding mounting holes and then, in turn, soldered to the associated trace(s). To ensure that a reliable electrical connection is made between each lead and its associated trace, the trace takes on a substantially circular appearance to form a so-called "pad" with the corresponding mounting hole at its center. Solder is then applied to cover the entire pad as well as the lead protruding through the mounting hole.
In some instances electrical components are situated on one side of the printed circuit board and the traces appear on the other. These boards, commonly referred to as a "single-sided" boards, are used primarily to implement relatively simple circuitry. Alternately, printed circuit boards may be fabricated with traces appearing on both sides. These boards are commonly referred to as "double-sided" boards and are finding substantially increasing use in fabricating relatively complex and/or dense circuits. In a double-sided board, the arrangement of traces appearing on either side is aligned with those appearing on the other side such that appropriate interconnections therebetween can be readily made through various suitable holes commonly referred to as "plated-through holes." In particular, the wall of each such hole is plated with a conductive material, usually copper, which electrically interconnects the pads appearing on both sides of the hole as well as providing an increased contact area between these pads and any corresponding component lead inserted through the plated-through hole.
Printed circuit boards can be manufactured using one or a combination of two well-known basic approaches; namely, the "subtractive" process and the "additive" process. As discussed in greater detail below, each process possesses unique drawbacks. In particular, the subtractive process is relatively expensive in that it requires the use of expensive photochemicals, resists and etchants and wastes substantial amounts of copper. In addition, the width of a trace is limited by a phenomena, described in detail below and commonly referred to as "undercutting," which limits the number of traces that can exist in any given area on a board; thereby disadvantageously restricting the overall circuit density that can exist on the board. While the additive process does not suffer from these deficiencies, this process, by contrast, generally produces very poor adhesion between the conductive foil comprising each trace and the insulating substrate. In fact, this adhesion is generally so low that very little force is required to lift any trace up off the board. Moreover a substantial amount of time, typically on the order of at least 28-40 hours, is disadvantageously required to manufacture a printed circuit board using the additive process.
Specifically, the subtractive process generally involves starting with a insulating substrate which has a relatively thick layer of copper laminated (clad) to one or both of its sides. Component mounting holes are then completely drilled through both the copper layer(s) and the substrate. Thereafter, a etch-resistant material (so-called "resist") typically in ink form, is applied usually by silk-screening or a similar technique, over each copper layer in a positive image of the desired circuit configuration to be fabricated thereon. Alternatively, to yield greater circuit density through increased resolution, i.e. narrow trace width, and uniform reduced spacing between adjacent traces, printed circuit board manufacturers are increasingly using photographic techniques. Here, a photo-sensitive resist (so-called "photo-resist") is uniformly disposed over each entire copper layer. An appropriate photographic transparency is then made in the form of a negative of the desired circuit configuration for each side of the board. Each side of the board is then exposed through its corresponding transparency to intense utra-violet light. Those regions of the photo-resist which were exposed to this light polymerize and become etch-resistant. Thus, a positive latent image of the desired circuitry is formed in the photo-resist after exposure. Once the desired circuit pattern is formed in the resist, regardless of the specific technique used, the board is then immersed in a copper etchant bath which removes (etchs away) all the copper which is not protected by the resist or polymerized photo-resist. Once the etching is complete, the board is removed from the bath and the remaining resist is removed leaving the laminated copper on each side in the form of the desired circuit configuration thereon.
The subtractive method disadvantageously possesses several drawbacks. Chief among them is undercutting, i.e. lateral undermining of the traces found on the board and situated under the resist. Since the laminated copper layer is relatively thick, a strong etchant and considerable time is usually required to completely remove the excess copper during etching. Consequently, once enough of this copper is etched away, the lateral edges of the copper situated under the resist become exposed and thereafter this latter copper, supposedly protected by the resist, will be laterally etched away as long as the board remains in the etchant bath. Thus, whenever all the excess copper has been completely etched away, a significant amount of copper from the desired traces will also have been removed. As a result, the cross-sectional area and the current carrying capacity of each trace will be disadvantageouly reduced. To compensate for any expected undercutting, printed circuit board designers must use wider traces, than would otherwise be required in the absence of undercutting, in order to achieve any desired current carrying capacity. Unfortunately, the increased size (width and cross-sectional area) greatly limits the resolution, i.e. minimum width, of a trace. This thus disadvantageously impedes continued miniaturization of the traces and that, in turn, limits the density of the circuitry that can be fabricated using printed circuit boards. Unfortunately, whenever the thickness of the copper clad must be substantial, e.g. to carry relatively high currents, the etchant time must be increased which disadvantageously increases the likelihood of undercutting, and, in the absence of employing significantly wider traces, the likelihood that a desired conductive trace can be completely removed by undercutting; thereby further restricting circuit miniaturization. The problem of undercutting has been repeatedly recognized in the art. See, for example, U.S. Pat. No. Re. 28,042 (issued to H. L. Rhodenizer et al on June 11, 1974); U.S. Pat. No. 3,620,933 (issued to J. J. Grunwald on Nov. 16, 1971); and B. L. Betteridge et al, "Copper Oxide Undercut (Haloing) During Wet Processing on Multilayer Circuit Boards," Proceeding PC' 81--Fourth International Printed Circuits Conference, New York, June 2-4, 1981, pp. 1-10.
The subtractive method possesses other drawbacks as well. For example, since the desired circuit configuration often occupies half of or, in most instances, much less than half of the total available area of each laminated copper layer, considerable copper is wasted during etching. While copper-rich etchant solutions can be processed to recover this copper, the associated expense is generally considerable and usually economically unprofitable unless an exceedingly large number of printed circuit boards are being manufactured. Since most manufacturers do not produce this many boards, copper-rich etchant is often discarded and a significant amount of copper is lost. Moreover, resists and particularly photo-resists tend to be quite expensive.
In an effort to eliminate wasted copper, the art has developed an alternate process, i.e. the so-called additive process, for manufacturing printed circuit boards. With this method, the only copper that is consumed is that needed to produce the desired traces on each printed circuit board. Specifically, the additive process starts merely with the unclad insulating substrate used for the printed circuit board. All the holes, which are to become plated-through holes are then drilled through the substrate. Thereafter, an activating agent which promotes the adhesion of copper to the unclad insulating substrate is generally applied over the entire substrate. A mask is then applied to the substrate, and the substrate is then, in turn, immersed in an electroless copper plating bath which deposits copper onto all the non-masked areas including the walls of the holes. Once the plating is completed, the board is removed from the bath and a protective coating (e.g. a solder mask) is applied over each plated side of the board.
Clearly, the additive process saves copper. Inasmuch as this process generally minimizes or totally eliminates the need for etchants, undercutting simply does not occur in the additive process. Thus, narrow traces can be made using this process which, in turn, advantageously permit denser circuits to be manufactured than those obtainable through the subtractive process. However, the additive process unfortunately possesses several significant drawbacks--substantial time, on the order of 28-40 hours, is disadvantageously required to electrolessly deposit a copper coating of sufficient thickness and, more significantly, once this coating is completely deposited, it adheres extremely poorly to the substrate. This poor adhesion causes the printed circuit, i.e. the plated copper traces, to easily "peel" off the substrate during subsequent manufacture and/or use of the board. Several attempts have been disclosed in the art for promoting adhesion between the copper and the subtrate; however, these attempts either result in only a marginal improvement in adhesion or are either too expensive or complex to implement in practice. See, for example, the processes disclosed in U.S. Pat. No. 4,253,875 (issued to K. Heymann et al on Mar. 3, 1981); U.S. Pat. No. 4,217,182 (issued to J. E. Cross on Aug. 12, 1980); U.S. Pat. No. 3,666,549 (issued to H. L. Rhodenizer et al on May 30, 1972) and reissued as U.S. Pat. No. Re. 28,042; U.S. Pat. No. 3,625,758 (issued to F. T. Stahl et al on Dec 7, 1971); and also M. J. Canestaro, "Additive Plating Coverage," IBM Technical Disclosure Bulletin, Vol. 20, No. 9, Feb. 1978, page 3391; A. Adler et al, "The Use of Catalytic Basis Materials and Ductile Electroless Copper Deposition in the Manufacture of Printed Wiring," Plating, Vol. 56, No. 8, Aug. 1969, Pages 880, 882, 884, 886 and 888; and N. Lonhoff, "Production of Printed Circuits by the Additive Techniques using Nibodur," Transactions of the Institute of Metal Finishing, Vol. 46, Spring 1968, Part I, Pages 194-198.
Thus, while the additive process offers substantial cost savings--through eliminating copper waste and minimizing the use of etchants--over the subtractive process as well as providing increased miniaturization capability, no truly effective, inexpensive and simple solution has yet been found in the art to the problem occurring in the additive process of providing substantial adhesion between a copper coating and a printed circuit board substrate. Inasmuch as the subtractive process usually provides adequate adhesion, manufacturers have to date primarily relied on this process to manufacture printed circuit boards even in spite of its drawbacks: high cost and restricted potential for increased miniaturization.